Switch ircd.conf to block-based configuration format

Switch ircd.conf to block-based configuration format

Replace the legacy colon-delimited config format with a block-based
format using curly braces and key-value pairs. This eliminates the
colon separator conflict with IPv6 addresses and supports multi-value
fields (e.g. multiple hosts in a single kline block).

The connect{} block merges the old C/N/H line triplet into a single
block. The allow{} block replaces I-lines with separate ip and host
keys. All other block types map directly to their legacy equivalents.

Includes a one-time converter script (tools/convert-conf.py) to
migrate existing configs, and a rewritten chkconf validator.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>